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Keeping cool without sweat

Note for new readers: this blog presents a rather technical approach to the subject of living comfortably in the tropics. It aims to teach you about tropical comfort and discomfort. Don't read this blog unless you got a "B" or better in high school physics or are an architect/builder who needs to understand heat transfer! If you're more interested in tropical lifestyle matters, the ups and downs of living in paradise, you can check out the blog by my brother Jack at:

http://www.nationmultimedia.com/webblog/home.php?uid=361

One of our readers, reflecting on the pleasures of keeping cool using one of those "mist fan" machines while struggling to enjoy a good cigar, raises the following question:

There has to be some balance point here, where body cooling by external evaporation is such that the body does not need to sweat heavily and so is happy in a high humidity atmosphere. These machines do work, the only problem I have with them is keeping my cigar alight...........

Yao Bar at sundown, your author making the sketch that you see here

Appropriate meditiation on this matter requires a proper setting, and I write here from one of my favorite streetside venues, the Yao Bar off Ekamai, where the sun is setting in a tawny sky at the end of a day warm enough to raise a slight sweat as I was walking here. Now the waitress is serving a large (22 oz) bottle of cold Chang beer, 70 baht, over a large glass of ice cubes (3 degrees C, but my radiant thermometer is probably mistaken here because it's mosquito brain does not really know the exact emssivivity of ice, or as you can see, of the sky which of course is not 6 degrees but closer to 30 degrees, only emitting like a 6 degree object). Here, carressed by a gentle breeze, watching life, much of it casually and lightly clothed as tends to be on these sois, I am comfortable enough, not sweating at all, though I am continuing to produce, as we all do while engaged in these indolent activities -- and subsisting on a healthy Thai diet of fish and rice, not fatty British pub food simultaneously engaged in heated discusion of such matters as cricket and motor racing-- heat. And my heat production is likely to be around 50 watts.

Now sitting here I perceive that I am comfortable but on the verge of sweating. (At this point I can only appeal to the reader's intuition when I say comfortable, since I can neither define that term or conversely define its opposite, uncomfortable, though in the future we will necessarily have to face this definitional task, not as simple as it might first appear).

Let us now proceed to make some estimates of the transfer of heat that is occurring at my bodily envelope (having disposed of the problem of dealing with transfer of moisture, since I'm not sweating, though almost sweating).

Since I am warmer than the air around me by about 3 degrees, and the transfer of heat being a linear function of temperature difference with a constant of proportionality of about 10 (for a pleasant breeze) my transfer via convection to the air will be in watts

W=10*(34-31)=30 watts

I am also radiating heat directly to both the "cold" sky at 6 degrees and my other surroundings, and without going through l the tiresome arithmetic, Stefan Boltzmann constants and so forth (unless I later add an appendix to this chapter), my estimate for radiant loss is a minor 10-15 watts.

The 22 ounce bottle of Chang beer which by ingesting I am raising from 18 to 37 degrees over a time period of 20 minutes provides further cooling of about 5 watts, not counting the vasodilative impact of alcohol on improving the blood supply to my nose. I neglect my cigar, and I am now too weary with these exertions to account for respiratory loss, either sensible or latent, though I think it might be as much as a few watts.

Observation of surroundings: the waitress is hacking at a mango in the course of making herself som tam dinner; though lightly dressed her skinny brown legs to which my eyes are drawn are doubtless more effective in transferring heat to the air than my ham-like euro limbs. The bartender, who has a large belly, is engaged in mild activities, and has elected for his comfort to be shirtless. Altogether, we represent a sample of three varied non-sweating bodies, with heat production approximately equal to the rate of heat being lost to our environment. Therefore we meet a working definition of comfort with sweating:

Energy produced sweatlessly=energy lost without need for sweat.

On this basis, I propose that for a typical fahlang, sweating will not be necessary for sedentary activity with surrounding temperature (e.g. air temperature but also consider surrounding radiant temperatures) of about 30 degrees or a little less. Under these or cooler conditions, we do not call on any heat dissipative mechanism that is sensitive to humidity, so we may say in a gross sense that humidity is not a factor in our comfort. If however our level of effort should increase to say 100 watts, we will not be able to throw off enough heat to the surroundings and after a few minutes our body and skin temperature will begin to rise and our sweat system will activate, with the rate of cooling being very sensitive to humidity (more on this later). And we will begin to say that we are too warm (suggesting that sweat = thermal discomfort.)

So my answer is this: humidity doesn't matter too much (except in relation to such issues as keeping cigars lit and beers cool) as long as we are not sweating, ie at a temprature of roughly 30 degrees in sedentary mode.

Thin Roofs...and foil

Even now at the winter solstice, with the noon solar radiation reduced some 20 percent, any thin darkish thin surface (such as a metal roof or a tile roof) heats up to 50 degrees C or more, with the heat being almost immediately transferred undiminished to the underside, as you can see from the temp reading of the underside of this rubberized canvas tent which is supposed to provide cooling shade but really provides heating shade which will radiate on the order of (52-32)*5=100 watts per square meter to my balding head, plus perhaps as much again convected to the air below. Not as bad as the 700 watt sun, you might say, but not comfortable either – and suffocating if the underside of the roof is not radically ventilated. So next time you are sitting under one of these tents at a Thai wedding or funeral, be warned -- it's the heat, not your emotions, that is causing you to feel like you are going to faint.

Here is an excerpt from the manual of the Australian company Bluescope Steel who make (and promote) steel roofing in Thailand. This indicates a good reduction in heat transmission through thin roofs (steel, cement tile, etc) with foil insulation, properly installed (air space above and below foil, and foil with shiny side down.) For a dark roof, the thermal transmission is said to drop from 165 to 22 watts, almost 90 percent.

Foil sales folks must be delighted with this conclusion. But be aware that it contains at least one hidden but important assumption -- very low convective transfer from the roof to the room below because the air beneath the foil is taken to be perfectly still. But this is not true in any case for many reasons. Convective cells develop even in sealed spaces, desirable fans move air, and required ventilation to replace that air which will otherwise heat at about 2 degrees per minute (ventilation rate of approx one room air change per minute--that's right, not one or two air changes per hour, as in most attic spaces even with vents) so there is in fact much more downward convective transfer from the roof than this chart suggests.

My estimates do not yield quite such promising results – my 1935 Eshbach Engineers Manual (college edition) indicates an R value bonus of only about 2 or so for a single layer of foil. And a direct measure of the efficacy of shiny vs. black spray paint surface on the foil which can be seen in the simple room simulation which I performed last month yields a reduction of about 50 percent in heat transmission. This implies that the heat flux from downward radiation and convection are about the same. Th foil brigade will try to tell you that here is not convective transfer downward at all, and this just ain't true.

Want to know the exact ruth? The fact is that the physics of downward heat flow in roofs is extremely complex, the stuff of learned papers by mathematically oriented professors.

But I don't question that properly installed foil will improve matters-- from very bad to not as bad--in the case of a thin tile, cement, or metal roof. Studies such as those done in Florida suggest aircon energy savings of perhaps 15 percent with the addition of foil. My caveat is that in most practical situations one cannot rely on foil alone to prevent uncomfortable heat buildup from a foil-insulated roof.

Some reasonably unbiased web sites describing use of foil insulation are:

Florida studies indicating reduction of energy 25 percent and temp 6 deg:

http://www.fsec.ucf.edu/en/publications/html/FSEC-CR-1231-01/index.htm

Wikipedia:

http://en.wikipedia.org/wiki/Building_insulation

Q&A on foil insulation by experienced researchers:

http://www.fsec.ucf.edu/en/publications/html/FSEC-EN-15/index.htm

Night before Christmas

Some think of Santa. I dream of Planck's Constant.

I put a steel bar outside on the roof at night, facing upward into the relatively clear sky (stars visible), compare its temp with the temp of a shielded sensor; you see, on December 23-24, 2007, a temp suppression of the bar of only about one degree. This is with a sky radiant temp (Radioshack sensor) of about 5 degrees and relatively high dew point of about 21 degrees. Notice the wave of humidity at about 2 to 4 am, dew point reaches close to air temp, so RH is close to 100 percent and the temp suppression becomes much less (in part because water begins to condense on the steel surface, creating heat.). Therefore we cannot get much radiant cooling under these circumstances of high air saturation.

It happens that I did not in my bed dreaming of sugar plums notice any suffocating wave of humidity at 2-4 am, perhaps it did not arise in the indoor environment. Why? Or was it the low temp, not over 23 degrees, that kept my body well below the point of perspiration? All worth investigating as the hot season approaches and the night temps hover up at around 30 rather than this month's more comfortable mid twenties.